"Science fiction writers, I am sorry to say, really do not know anything. We can't talk about science, because our knowledge of it is limited and unofficial, and usually our fiction is dreadful."
- Philip K. Dick

Several ships are attacking a stationary installation on the Moon. The ships have the benefit of mobility in attacking a fixed target; what advantage could the defenders of the installation have that could turn the tide of battle?

Note: this is a different version of the weapon from that used in Clarke's original version of the story. See the article describing the polaron beam from Clarke's original 1951 novella Earthlight, published in Wonder Stories.

Wheeler never knew why the fortress waited so long before it used its main weapon. Perhaps Stephanson, or whoever was in charge, was waiting for the attack to slacken so that he could risk lowering the defenses of the dome for the millisecond that he needed to launch his stiletto.

Wheeler saw it strike upward, a solid bar of light stabbing at the stars. He remembered the rumors that had gone around the Observatory. So this was what had been seen, flashing above the mountains. He did not have time to reflect on the staggering violation of the laws of optics which this phenomenon implied, for he was staring at the ruined ship above his head. The beam had gone through Lethe as if she did not exist; the fortress had speared her as an entomologist pierces a butterfly with a pin.

I love this story, and I avoided putting this item on the site for a long time. However, now that DARPA is working on it, I'll put it up. **SPOILER ALERT**

Clarke puts the problem this way:

"...It looked like a beam of some kind, but of course that's impossible. No beam can be visible in a vacuum."

The answer, of course, is that it only looked like a beam of light.

The answer is provided to the reader:

He could well appreciate why a jet of molten metal, hurled through space at several hundred kilometers a second by the most powerful electromagnets ever built, might have looked like a beam of light flashing on for an instant.